Non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, have been used widely as the batteries to be used for personal computers, cellular phones, personal digital assistant devices, etc. due to their high energy densities.
Such non-aqueous electrolyte secondary batteries typified by lithium secondary batteries are high in energy density and they may generate heat due to the occurrence of high current when internal short-circuit or external short-circuit has been caused, for example, by damage to a battery or damage to an instrument using a battery therein. Therefore, non-aqueous electrolyte secondary batteries have been demanded to prevent generation of a certain amount or more of heat and secure high safety.
A method of imparting a shut-down function to prevent further heat generation by blocking the passage of ions between the positive and negative electrodes with a separator at the time of abnormal heat generation is common as means for securing safety. One known technique for providing a separator with a shut-down function is a technique of using a porous film made of a material that melts on abnormal heat generation. In a battery using this separator, the porous film melts and closes pores on abnormal heat generation to block the passage of ions and thereby can suppress further generation of heat.
As a separator having such a shut-down function, a porous film made of a polyolefin is used, for example. A separator made of the porous polyolefin film melts and closes pores at about 80 to 180° C. on abnormal heat generation of a battery to block (shut-down) the passage of ions and thereby suppress further generation of heat. However, in some cases, a separator made of the porous film allows a positive electrode and a negative electrode to come into direct contact with each other due to the shrinkage, rupture, or the like thereof, resulting in the occurrence of short circuit. A separator made of a polyolefin porous film may have insufficient shape stability and may be unable to suppress abnormal heat generation caused by short circuit.
Some non-aqueous electrolyte secondary battery separators excellent in shape stability at high temperatures have been proposed. One of the measures proposed to that end is a non-aqueous electrolyte secondary battery separator composed of a laminated porous film in which a heat-resistant layer comprising a filler is laminated with a porous film mainly comprising a polyolefin to serve as a substrate (the porous film is hereinafter sometimes referred to as a “porous film substrate”) (see, for example, Patent Document 1). One of the problems with such a separator is preventing the filler from coming away from the surface of the laminated porous film.
In a case that a filler comes away from a separator, some defects may occur such as failure to develop properties expected as a separator and pollution of an apparatus with a powder (filler) that has come away during the fabrication of a battery.
As a method for preventing a filler from coming away, there have been proposed a method in which the surface of the filler is modified (see, for example, Patent Document 2), a method in which the chemical structure of the binder resin to which the filler is to be bound is characterized (see, for example, Patent Document 3), and a method in which the average fiber diameter of the fibers to which the filler is to be fixed and the particle diameter of the filler are controlled to have a prescribed relation (see, for example, Patent Document 4).
However, these methods insufficiently can prevent a filler from coming away, and further improvement has been demanded.